The present invention relates to a magnetic tape reel for a magnetic tape cassette.
Heretofore, in a video tape cassette, the magnetic tape is wound on a pair of magnetic tape reels and is incorporated in the cassette case. In these cassettes, a magnetic tape winding drum is made up of an inner cylinder coupled to a rotating shaft, an outer cylinder disposed outside the inner cylinder and providing a tape winding cylindrical wall, and a plurality of reinforcing ribs extending radially of the reel to connect the outer cylinder and the inner cylinder. The tape winding drum has upper and lower flanges to prevent the magnetic tape from being wound irregularly.
The reinforcing ribs extending radially of the reel are necessary for increasing the mechanical strength of the tape winding drum and for facilitating the flow of molten plastic resin in molding the reel. However, the resin contracts immediately after the molding of the reel, and thus the ribs tend to be contracted towards the center axis of the reel more than the other parts. As a result, the circularity or cylindricity of the tape winding drum is often less than desired.
In order to overcome the above-described difficulty, there have been proposed a variety of reels. For instance, in a reel disclosed by Japanese Published Utility Model Application No. 28313/1982, two spaces define an S shape in a horizontal section of the inner cylinder coupled to the rotating shaft and the outer cylinder providing the tape winding cylindrical wall. That is, the outer cylinder is of a dual structure having a certain space, and the reinforcing ribs are connected to the inner wall of the outer cylinder, but are not connected directly to the outer wall forming the tape winding cylindrical wall. Accordingly, the contraction of the reinforcing ribs during cooling of the molten resin is absorbed by the inner wall, whereby the circularity of the tape winding drum is maintained.
If the reel disclosed by Japanese Published Utility Model Application No. 28313/1982 is made of a resin such as acrylonitrile butadiene styrene (ABS) resin which has a relatively small contraction when cooled, the effect is sufficient. However, if it is made of a resin such as polyacetal (POM), there is still a problem.